1. Field of the Invention
The present invention relates to a ring network system in which a plurality of devices provided in a vehicle are connected in a ring-like manner by a transmission line to mutually effect data communication among the devices. More particularly, the present invention relates to a ring network system which facilitates addition of devices to expand the network and which permits addition of a device not having a communication integrated circuit to the network.
The present application is based on Japanese Patent Application No. Hei. 10-193422, which is incorporated herein by reference.
2. Description of the Related Art
A block diagram of the related ring network system provided in a vehicle is shown in FIG. 13. As shown in FIG. 13, the ring network system is configured such that a device 201a having a communication integrated circuit (communication IC) 203a for controlling the transmission and reception of data, a device 201b having a communication IC 203b, and a device 201c having a communication IC 203c are respectively connected in a ring-like manner by a transmission line 205.
These devices are devices such as a radio tuner, an amplifier, a compact disk player (hereafter referred to as the CD player), a cassette deck, and the like, and data communication is mutually effected among the devices. For example, a CD signal can be sent to the amplifier through the transmission line by operating the CD player, and the CD signal can be amplified by the amplifier, and the sound can be outputted from a speaker.
In addition, in a case where the ring network system for effecting data communication at high speed is configured, transmission lines such as optical fibers capable of high-speed data transmission are used.
However, with the ring network system shown in FIG. 13, the transmission lines such as the optical fibers are required for connecting the adjacent devices, and it has been necessary to provide an optical connector for each device. For this reason, since the configuration of the ring network system becomes complex, it has been difficult to further add devices to the network and expand the network.
Furthermore, with an ordinary ring network system, since the configuration provided is such that the devices having the communication ICs are connected to the transmission line, a non-network device (analog device) which does not have a communication IC is extremely difficult to add to the same ring network system.
Meanwhile, a ring network system such as the one shown in FIG. 14 is configured such that a first device 207a having a communication IC 209a and a second device 207b having a communication IC 209b are provided as standard equipment, and are connected in a ring-like manner by transmission lines 211a and 211b. 
In this case, the positions of the first device 207a and the second device 207b, the wiring routes of the transmission lines 211a and 211b, the length of the transmission lines, and the like are already set in many cases. For this reason, as shown in FIG. 15, if a third device 207c is newly added to the ring network system, it has been necessary to further provide a relay connector 213 and transmission lines 211c and 211d. 
In addition, in the case where optical fibers are used, a light signal is substantially attenuated, and waveform distortion, jitter, and the like occur due to the relay connector 213. For this reason, there has been a problem in that normal communication cannot be effected.
Accordingly, a primary object of the present invention is to provide a ring network system which is capable of effecting addition of devices and expansion of the network easily with a simple configuration.
A secondary object of the present invention is to provide a ring network system which is capable of effecting addition of devices and expansion of the network without changing the positions of the existing devices and the routing forms of the existing transmission lines.
To this end, in accordance with a first aspect of the present invention, there is provided a ring network system in which a plurality of network devices each having a first communication control unit for controlling data communication are connected in a ring-like manner by a transmission line to mutually effect transmission and reception of data among the network devices, comprising: an addition unit disposed between two adjacent ones of said plurality of network devices, said addition unit and said plurality of network devices being connected in the ring-like manner by said transmission line, wherein said addition unit has a plurality of device inserting slots in which a plurality of additional devices are installed and which are provided in correspondence with said plurality of additional devices, and a second communication control unit for controlling data communication among said plurality of additional devices installed in said plurality of device inserting slots and for controlling data communication between said additional device and said network device.
In accordance with the above-described first aspect of the invention, if a plurality of additional devices are installed in the plurality of device inserting slots provided in the addition unit, data communication can be effected among the plurality of additional devices installed in the plurality of device inserting slots, and data communication can be effected between each additional device and said network device. Namely, by merely installing the additional device in the device inserting slot, the system can be configured automatically, and devices can be easily added to expand the network.
In accordance with a second aspect of the present invention, said plurality of additional devices may be said network devices each having said first communication control unit. Alternatively, in accordance with a third aspect of the present invention, said plurality of additional devices may be non-network devices each not having said first communication control unit.
In accordance with a fourth aspect of the present invention, the ring network system according to the first aspect of the invention further comprises: a device determining unit which, on the basis of a signal sent from said additional device installed in said device inserting slot, determines whether said additional device is said network device having said first communication control unit or a non-network device not having said first communication control unit.
In accordance with the above-described fourth aspect of the invention, since the device determining unit determines whether said additional device is said network device having said first communication control unit or a non-network device not having said first communication control unit, it is possible to control the installed device according to the network device and the non-network device.
In accordance with a fifth aspect of the present invention, in the ring network system according to the fourth aspect of the invention, said second communication control unit includes: a first selecting portion which, if said device determining unit has determined that said additional device is said non-network device, selects said non-network device on the basis of a result of the determination; a second selecting portion which, if said device determining unit has determined that said additional device is said network device, selects said network device on the basis of a result of the determination; a converting portion for converting a signal in a first form from said non-network device selected by said first selecting portion into a signal in a second form of said network device; and a communication portion for transmitting the signal in the second form converted by said converting portion to one of said plurality of network devices.
In accordance with the above-described fifth aspect of the invention, if it is determined that the additional device is a non-network device, the first selecting portion. selects the non-network device, whereas if it is determined that the additional device is a network device, the second selecting portion selects the non-network device.
Then, if the converting portion converts a signal in a first form from said non-network device selected by said first selecting portion into a signal in a second form of said network device, the communication portion transmits the signal in the second form to one of said plurality of network devices, so that communication can be effected from the non-network device to the network device.
In accordance with a sixth aspect of the present invention, in the ring network system according to the fifth aspect of the invention, said communication portion receives the signal in the second form from said network device on said transmission line, and outputs the same to said converting portion; said converting portion converts the signal in the second form from said communication portion into the signal in the first form; and said first selecting portion outputs the signal in the first form from said converting portion to said non-network device installed in said device inserting slot.
In accordance with the above-described sixth aspect of the invention, since said communication portion receives the signal in the second form from said network device on said transmission line and outputs the same to said converting portion, said converting portion converts the signal in the second form from said communication portion into the signal in the first form, and said first selecting portion outputs the signal in the first form from said converting portion to said non-network device installed in said device inserting slot, communication from the network device to the non-network device installed in the device inserting slot can be effected.
In accordance with a seventh aspect of the present invention, the ring network system according to the first aspect of the invention further comprises: an installation determining unit which, on the basis of the signal sent from said additional device, determines whether or not said additional device has been installed in said device inserting slot.
In accordance with the above-described seventh aspect of the invention, on the basis of the signal sent from said additional device, it is possible to determine whether or not said additional device has been installed in said device inserting slot.
In accordance with an eighth aspect of the present invention, there is provided a ring network system in which a plurality of devices each having a communication control unit for controlling data communication are connected in a ring-like manner by a first transmission line to mutually effect transmission and reception of data among the devices, comprising: an additional device having said communication control unit, a second transmission line, and a waveform shaping device, wherein said plurality of devices, said additional device, and said waveform shaping device are connected in the ring-like manner by said first transmission line and said second transmission line, and said waveform shaping device shapes the waveforms of signals included in the data on said first transmission line and said second transmission line and amplifies the signals.
In accordance with the above-described eighth aspect of the invention, in a case where an additional device is to be added to the network having a plurality of existing devices, by merely providing the second transmission line and the waveform shaping device it is possible to configure the ring network system without changing the positions of the plurality of existing devices and the wiring route and length of the first transmission line.